This invention relates to a braking control apparatus for an electric motor operated vehicle, and more particularly, to a braking control apparatus which is suitable for use in a battery operated fork lift.
In the battery operated fork lift, it has hitherto been the general practice to utilize a plugging operation as the means for electric braking. The reason for adoption of this system is that in order to perform such electric braking, it is not especially necessary to change the connection of the main circuit, and that upon completion of electric braking, it is possible to directly and continuously shift to operation in the reverse direction.
The fork lift frequently repeats forward and backward movements in the process of loading and unloading of goods. In particular, it is common pratice during the operation of such a vehicle that, while turning back the steering wheel in order to change the direction of vehicle movement, shifting from backward movement to forward movement is simultaneously performed. The plugging operation is an electric braking operation which is well adapted for this type of running operation, which is peculiar to the fork lift. The system, however, is inferior in efficiency because all the braking energy is merely dissipated as heat generated in the internal resistance of the armature by current flowing through a plugging diode and no regenerative current can be caused to flow to the battery as a result of the electromotive force induced in the armature.
Further, there are the problems of the deterioration of insulation, the cooling, etc., ascribable to the generation of heat in the armature. Accordingly, the motor inevitably becomes expensive to manufacture. For such reasons, the plugging operation is not employed in the braking of railway vehicles, electromobiles, etc., the regenerative braking system being exclusively adopted for such vehicles.
The regenerative braking system, however, requires a large number of contactors or thyristors (silicon controlled rectifiers) for the change-over between the powering phase and the regeneration braking phase, and therefore, this type of system also is very expensive. In addition, a regenerative braking system is unsuitable for the running characteristic of a fork lift since the forward power running cannot be initiated continuously from backward movement. Even if the main circuit is quickly switched when the powering in the reverse direction is to be shifted to upon completion of a braking period, a shock will arise when the switching timing is early. The main circuit must therefore be switched when the revolution of the motor has just stopped. It is apparent that the attachment of such a control device will render the system still more expensive.